All living cells must transport essential solutes across a semi-permeable membrane. A central pathway for the uptake of many different types of hydrophilic solutes is via membrane bound proteins that function as cation/solute cotransporters or symporters. These include uptake systems for sugars, amino acids, inorganic ions, and other small molecules. Human genetic diseases, such as type I cystinuria, involve defects in Na+/solute cotransporters. The broad aim of the proposed research is to understand the relationship between the biochemical structure of the H+/lactose permease found in Escherichia coli and its molecular mechanism of cotransporting H+ and lactose across the membrane. Our work utilizes FT-IR spectroscopy, site-directed mutagenesis, suppressor mutants, truncated versions of the lactose permease, and biochemical labeling methods. The first two aims use difference FT-IR spectroscopy to understand the mechanism of H+/lactose coupling and the nature of conformational changes associated with transport. Aims three and four complement this spectroscopy work by the analysis of site-directed mutations and suppressor mutations. A fifth aim will determine if two halves of the lactose permease work together to facilitate lactose transport. And finally a sixth aim will determine if a conserved motif plays a role in the positioning of transmembrane segments in the lactose permease. Ultimately, it is hoped that the molecular features of the lactose permease will be of general significance so that our results can be extended to provide a better understanding of other cation/solute cotransport systems in bacteria, fungi, plant, and animal cells.